1. Field of the Invention
The present invention relates to exposure by charged particle beam in the manufacturing process of a semiconductor device and a generator for generating exposure data to manufacture exposure masks or reticles and a method thereof.
2. Description of the Related Art
In recent years, because required pattern sizes have become miniaturized with the improvement of semiconductor device accumulation, sufficient resolution cannot be obtained through conventional exposure methods, and the formation of miniaturized patterns has become difficult. Therefore, exposure methods employing charged particle beam, electron beam in particular, are being currently used.
As electron beam exposure methods, there are: the point beam exposure method wherein resolution is high, but throughput low; the variable forming exposure method wherein throughput is improved by exposing pattern in small rectangular units; the segment one-shot exposure method wherein patterns repeatedly appearing within a chip are collectively lithographed by using a stencil mask; and the projection-type exposure method wherein the masks for all patterns are created and a large area is collectively lithographed, as in exposure by light. Because the shot number of electron beams can be reduced in the segment one-shot exposure method and the projection-type exposure method, throughput can be improved.
When a circuit pattern is plotted by radiating an electron beam onto a substrate resist film, the electron beam is radiated only onto the area of the pattern to be plotted. At this time, the electron incident on the resist film is partially forward-scattered, and the electron beam which permeates the resist film is partially back-scattered and is again incident on the resist film. Therefore, its influence propagates even if the electron beam is incident on one point on the resist film, thereby causing the generation of a so-called proximity effect.
As the energy intensity distribution (EID) function f(x, y) of the resist film when the electron beam is incident on one point on the resist film, the following formula wherein the term of forward-scattering and the term of back-scattering are each approximated by Gaussian distribution function is implemented:
                              f          ⁡                      (                          x              ,              y                        )                          =                              1                          π              ⁡                              (                                  1                  +                  η                                )                                              ⁢                      {                                                            1                                      β                    f                    2                                                  ⁢                exp                ⁢                                                                  ⁢                                  (                                      -                                                                                            x                          2                                                +                                                  y                          2                                                                                            β                        f                        2                                                                              )                                            +                                                η                                      β                    b                    2                                                  ⁢                exp                ⁢                                                                  ⁢                                  (                                      -                                                                                            x                          2                                                +                                                  y                          2                                                                                            β                        b                        2                                                                              )                                                      }                                              (        1        )            
Here, βf is the forward-scattering length, η is the back-scattering ratio, and βb is the back-scattering length. In addition, the first term in Formula (1) is called the forward-scattering term, and the second term thereof is called the back-scattering term. Forward-scattering has great influence on a narrow range, and back-scattering has a relatively small influence to a broad range. The ratio of these influences is η. These values are dependent on the energy of the electron beam, the thickness of the resist film, and the material of the substrate or the like, and are determined through experiments. The higher the acceleration voltage of the electron beam becomes, the smaller βf and the larger βb becomes. For example, if acceleration voltage is 100 kV and resist film thickness is 200 nm, βf is approximately 7 nm and βb is approximately 30 μm.
As correction methods for proximity effect in conventional exposure data generation, a simple and high-speed method which focuses attention on the differences between the influence ranges of forward-scattering and back-scattering is known (for example, refer to Patent References 1 and 2, below). In this method, the dimensions of pattern figures contained in exposure data are changed, taking into account the influence of proximity effect.
In instances of high acceleration voltage, it is unnecessary to consider the influence of forward-scattering from adjacent patterns because the influence range of forward-scattering is very narrow, and in addition, the influence of back-scattering can be approximated by small area units because the influence range of back-scattering is very broad. In Patent References 1 and 2, an area density map method is used as the approximation method for the latter.
The area density map method is a method wherein, as described in the following Non-Patent Reference 1, effectual area density is determined for each correction calculation mesh region by dividing exposure data into the correction calculation mesh regions, determining pattern area density within each correction calculation mesh region, and smoothing the area density according to the contribution of back-scattering. Here, pattern area density refers to the percentage of pattern area within one correction calculation mesh region to the entire area of the region.    Patent Reference 1: Japanese Patent Application Publication No. 2001-052999    Patent Reference 2: Japanese Patent Application Publication No. 2002-313693    Non-Patent Reference 1: F. Murai et al., “Fast proximity effect correction method using a pattern area density map”, J. Vac. Sci. Technol. B, Vol. 10, No. 6, pp. 3072-3076 (1992)
However, the afore-mentioned conventional exposure data generation methods have the following problems:
In a high acceleration voltage projection-type exposure method, the effectual influence of forward-scattering spreads due to blurring of the beam caused by aberration or coulomb effect. In particular, if current intensity is increased to improve throughput, the blurring of the beam is increased. Therefore, even in a high acceleration voltage exposure device, the overlapping of forward-scatterings from adjacent patterns is becoming difficult to ignore. In addition, in a low acceleration voltage exposure method, forward-scatterings are prone to overlap because the spreading of forward-scattering is relatively wide. Thus, although it is becoming vital to take into account the overlapping of forward-scattering, the problem lies in that search for adjacent patterns takes time.
Furthermore, if back-scattering intensity distribution is calculated by area density map method, the influence of back-scattering from patterns in the vicinity is also approximated. Therefore, if the patterns become increasingly miniaturized, errors caused by the approximation will become difficult to ignore.
In addition, if heavy metals such as copper and tungsten are present in the lower layer, as in multilayer wiring layers, its effect inevitably appears in the correction accuracy of exposure data as calculation errors when the correction calculation mesh region is large, because back-scattering intensity distributions with narrow spreading are intermixed. Although, in any case, the problem may be solved if the correction calculation mesh region is minimized, in this case, high-speed performance is sacrificed because more processing time is required.